This invention relates generally to testing of electronic circuitry, and more particularly concerns spring probes used in such testing.
In order to electrically test an electronic assembly, it is necessary to interconnect test points or circuits to appropriate connections on a test set. In testing printed circuit boards and other wired assemblies, this interconnection is usually accomplished through a "probing" operation. In its simplest form, probing is accomplished when a wire or probe is placed on a desired test point and a small amount of pressure is applied. Electrical contact is established and current flows through the probe to the test set and test results are observed.
In order to assure cost effective use of automatic test equipment (ATE), it is often required to make hundreds or thousands of test point interconnections within a few seconds. This is accomplished through use of a small device called a spring probe. As its name implies, a spring probe contains a spring and is used to probe electronic circuits. Basically, the spring mechanism allows the probe tip or plunger to be slightly displaced into the body of the probe when the test object is placed against the probe tip. Such spring action accomplishes two things:
(1) The spring tension applies the proper amount of force (2 to 4 oz. for example) to establish and maintain electrical connection with the contacted surface, and PA1 (2) The probe tip displacement (0.125 inch for example) compensates for small irregularities in the surface area contacted. PA1 (1) the plunger shaft is extended down and inside a sleeve or guide, and PA1 (2) the method of captivating the parts inside an enclosure is such that the mechanism is easily dismantled and reassembled.
As can be seen, any number of spring probes can be fixtured to any desired pattern and all can be engaged or disengaged through a single operation.
Spring probes are typically mounted in an insulator board in a pattern that corresponds to the geometry of the test object. Each desired test point has a correspondly probe and all probes are wired through a connector arrangement to a test set. The insulator board, spring probes and associated wiring are referred to as a "test fixture" because the pattern is fixed to the geometry of a particular test object. In general, insulator boards are removeable within a fixture so that multiple products can be tested with a common test set.
Due to electrical and mechanical failures, and occasional damage, it is highly desirable and usually required that spring probes be replaceable within a fixture. With present designs this is accomplished using a separate assembly called a socket or receptacle. Sockets are formed such that a wire or wire terminal can be attached to one end while the other end serves as a receptacle for a spring probe. Sockets are wired and become permanent to the fixture, whereas the spring probes do not. Due to present methods of construction, spring probe assemblies are non-repairable and are usually discarded when replaced.
Most manufacturers also offer what is called a "non-replaceable" spring probe that incorporates a probe/socket combination in a single enclosure. The design is identical to the replaceable probe, except that provisions are made for attaching a wire or wire terminal directly to the probe's enclosure. This version has cost advantages (one assembly vs. two) over the replaceable design, but has a decided disadvantage in maintenance. When failures or damages occur, the user must go to considerable expense and time delay in repairing the fixture. Wires must be disconnected, assemblies knocked or drilled out, etc. Due to the small parts and close tolerances, repair work frequently results in more failures induced than corrected. For this reason, the replaceable probe is more in demand than the less expensive non-replaceable version.